1. Field of the Invention
The present invention relates to dispersion and slope compensating optical fibers and transmission links for wavelength division multiplexing (WDM) systems, and more particularly to optical fibers and transmission links including such fibers that are particularly well suited for compensating dispersion and slope of Single Mode Fiber (SMF) operating in the C-band.
2. Technical Background
To meet the ongoing drive for more bandwidth at lower costs, telecommunications system designers are turning to high channel count dense wavelength division multiplexing (DWDM) architectures, longer reach systems and higher transmission bit rates. This evolution makes chromatic dispersion management critical to system performance, as system designers now desire the ability to accurately compensate dispersion across entire channel plans. Today, the only viable broadband commercial technology to battle dispersion has been Dispersion Compensating Modules (DCMs), i.e., spools having a suitable length of Dispersion Compensating Fiber (DCF) wound thereon. As DWDM deployments increase to 16, 32, 40 and more channels, broadband dispersion compensating products are even more desirable. Many current telecommunications systems have SMFs that, although they are optimized for zero dispersion at about 1310 nm, can also be utilized effectively to transmit signals at wavelengths around 1550 nm. This enables erbium-doped fiber amplifiers to be employed. An example of such a SMF is SMF-28(trademark) manufactured by Corning Incorporated. Prior Art FIG. 2 illustrates the refractive index profile for such a SMF. Typically, such fibers exhibit a dispersion of about 17 ps/(nmxc2x7km) and a dispersion slope of about 0.058 ps/(nm2xc2x7km) at 1550 nm.
With continuing interest in higher bit rate systems ( greater than 10 Gbs), long reach systems (e.g.,  greater than 500 km) and optical networking, it is imperative to use DCFs in networks that carry data on SMF as well. High bit rates, longer reaches and wider bandwidths require dispersion, but also dispersion slope to be compensated for more exactly.
Consequently, it is desirable for the DCF to have dispersion characteristics such that its dispersion and dispersion slope are matched to that of the SMF transmission fiber it is required to compensate. The ratio of dispersion to dispersion slope at a given wavelength is referred to as xe2x80x9ckappa (xcexa).xe2x80x9d Kappa changes as a function of wavelength for a given transmission fiber. Hence, it is equally important that the kappa value of the DCF is matched to that of the transmission fiber in the operating window.
It would be desirable to develop alternative DCFs, in particular, ones having the ability to compensate for dispersion of SMF over a wide wavelength band around 1550 nm.
The present invention is a dispersion compensating optical fiber which comprises a core refractive index profile which is selected to result in a fiber which exhibits negative dispersion and negative dispersion slope at 1546 nm and preferably exhibits low bend loss and low attenuation. The DCF in accordance with the present invention is particularly effective at compensating for both the dispersion and slope of a SMF in a transmission link operating within the C-band operating window. More particularly, the present invention is a DCF comprising a core refractive index profile with a central core segment having a positive relative refractive index xcex941, a moat segment surrounding the central core segment having negative relative refractive index xcex942, and a ring segment which surrounds the moat segment having a positive relative refractive index xcex943, wherein xcex941 greater than xcex943 greater than xcex942,
And where xcex94 is defined as:   Δ  =                    (                              n            1            2                    -                      n            c            2                          )                    2        ⁢                  n          1          2                      xc3x97    100.  
The DCF in accordance with a first embodiment of the invention exhibits a core refractive index profile that results in a negative dispersion slope of less than xe2x88x920.29 ps/(nm2xc2x7km) at a wavelength of 1546 nm, a negative dispersion of between xe2x88x92100 ps/(nmxc2x7km) and xe2x88x92120 ps/(nmxc2x7km) at a wavelength of 1546 nm, and a kappa value obtained by dividing the dispersion by the dispersion slope at 1546 nm in the range between of 250 to 387 nm. The DCF preferably has a cladding layer surrounding the ring segment and having a relative refractive index xcex94c, wherein xcex941 greater than xcex943 greater than xcex94c greater than xcex942.
The DCF in accordance with another embodiment of the invention exhibits a core refractive index profile that results in a negative dispersion slope of less than xe2x88x920.29 ps/(nm2xc2x7km) and greater than xe2x88x920.40 ps/(nm2xc2x7km) at a wavelength of 1546 nm, and more preferably less than xe2x88x920.36 and greater than xe2x88x920.40 ps/(nm2xc2x7km) at 1546 nm. In accordance with the invention, the DCF also exhibits a negative dispersion of between xe2x88x92100 ps/(nmxc2x7km) and xe2x88x92120 ps/(nmxc2x7km) at a wavelength of 1546 nm, and more preferably between xe2x88x92105 ps/(nmxc2x7km) and xe2x88x92120 ps/(nmxc2x7km) at 1546 nm. The DCF in accordance with the invention preferably exhibits a kappa value obtained by dividing the dispersion by the dispersion slope at 1546 nm in the range between of 250 to 387 nm. The DCF preferably has a cladding layer surrounding the ring segment and having a relative refractive index xcex94c, wherein xcex941 greater than xcex943 greater than xcex94c greater than xcex942.
Advantageously, the cutoff wavelength (xcexc) of the DCF is less than 1500 nm and more preferably less than 1350 nm. Low cutoff wavelength in a DCF is advantageous because it provides a system that may only propagate light in the fundamental mode. Thus, Multiple Path Interference (MPI) may be significantly reduces which, therefore, reduces system noise in the C-band wavelength window.
In accordance with another embodiment of the DCF of the present invention, the peak delta xcex941 of the central core segment is greater than 1.6% and less than 2.0%, and more preferably greater than 1.7% and less than 1.9%. The lowest delta xcex942 of the moat segment is less than xe2x88x920.25% and greater than xe2x88x920.44%, and is more preferably less than xe2x88x920.30% and greater than xe2x88x920.37%. The peak delta xcex943 of the ring segment is greater than 0.2% and less than 0.5%, and more preferably greater than 0.35% and less than 0.45%.
In accordance with another embodiment of the invention, the dispersion compensating optical fiber has an outer radius r1 of the central core segment between 1.5 and 2 microns; an outer radius r2 of the moat segment between 4 and 5 microns; and a center radius r3 of the ring segment between 5.5 and 7 microns. More preferably, the outer radius r1 of the central core segment is between 1.6 and 1.8 microns; the outer radius r2 of the moat segment is between 4.2 and 4.8 microns; and the center radius r3 of the ring segment is between 6 and 6.5 microns.
According to another embodiment of the invention, the dispersion compensating optical fiber has a core/moat ratio, taken as r1/r2, that is greater than 0.34 and less than 0.40 and an effective area (Aeff) at 1546 nm that is greater than 18 square microns, and more preferably greater than 20 square microns. This large effective area is desirable as it can reduce non-linear effects. The DCF preferably exhibits an attenuation of less than 0.6 dB/km at 1550 nm thereby not appreciably adding to the total attenuation of the transmission link. Additionally, the DCF preferably exhibits a bend loss that is less than 0.01 dB/m, and more preferably less than 0.005 dB/m at 1550 nm on a 40 mm diameter mandrel. Low bend loss is very important in DCF""s as it allows for compact packaging on the modules and helps to keep the total link attenuation low.
In accordance with a preferred embodiment of the invention, the DCF includes a ring segment having a lower delta tail portion that meets the zero delta % at a radius greater than 8 microns, more preferably greater than 10 microns, and most preferably greater than 12 microns. The tail portion 40 preferably has a delta % of greater than 0.02% and less than 0.2% at a radius between 7 and 8 microns. Preferably, the tail portion tapers linearly from about 8 microns to the zero delta % 42.
In another embodiment of the invention, an optical fiber transmission link is provided comprising a length of SMF optimized for low dispersion operation at a wavelength range of 1290 nm to 1320 nm; and a length of DCF having a dispersion value between xe2x88x92100 and xe2x88x92120 at 1546 nm wherein within a transmission band of between 1520 nm to 1570 nm the transmission link exhibits an absolute value of residual dispersion less than about 0.15 ps/km/nm. Preferably, the length of SMF is greater than 6 times, and more preferably greater than 7 times, the length of the length of DCF.
Additional features and advantages of the invention will be set forth in the detailed description which follows, and will be readily apparent to those skilled in the art from that description or recognized by practicing the invention as described herein, including the detailed description which follows, the claims, as well as the appended drawings.
It is to be understood that both the foregoing general description and the following detailed description are merely exemplary of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed. The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate various embodiments of the invention, and together with the description serve to explain the principles and operation of the invention.